A conventional image reading apparatus has two different reading systems, i.e., a reading system in which an original is fixed on an original glass plate and an optical system scans it to read image information, and a reading system in which an optical system is fixed and an original is moved so image information is read from it.
FIG. 3 is a view showing the arrangement of a conventional image reading apparatus, and shows a reading system in which an optical system is fixed and image information of an original 120 is read while the original 120 is conveyed by a DF (abbreviation for a document feeder), although not shown. The original 120 is moved by the DF. When the original 120 passes above a scan flow glass member 104, it is illuminated through a light source 106 and reflecting member 107. A light beam reflected by the original 120 is transmitted through a slit 108 and forms an image on a line sensor 113 such as a CCD by an imaging lens 112 through first, second, and third mirrors 109, 110, and 111. As the original 120 moves at a constant speed, the line sensor 113 such as a CCD reads it at a predetermined timing, so the image information of the original 120 can be read.
When an original 130 is placed on an original glass plate 101, while it is illuminated by the light source 106 and reflecting member 107 of an illuminating portion, the light source 106, reflecting member 107, slit 108, and first mirror 109 cooperate with each other to scan it in the subscanning direction indicated by an arrow X in FIG. 3. At the same time, the second and third mirrors 110 and 111 move in the subscanning direction indicated by the arrow X. Thus, the image information on the original 130 is read by the line sensor 113 such as a CCD.
When an image reading apparatus is formed with the above arrangement, an original such as a book, a thin original which is difficult to convey with the DF, or the like can be read if it is placed on the original glass plate 101. Simultaneously, a sheet-type original can be read while it is moved on the scan flow glass member 104. In particular, if the original can be read by the latter scan flow scheme, it can be read quietly because it is not scanned by an optical system, and can be read at a high speed.
In the prior art described above, the optical system is fixed and the original is read while being conveyed, that is, so-called scan flow operation is performed, so the advantages such as quietness and higher speed are realized. However, due to dust attaching to the scan flow glass member 104, or a scar on the glass plate itself, the following problems arise.
Currently, the above image reading apparatus generally has a reading density of about 400 dpi to 600 dpi. In this case, a pixel with a size of 63.5 μm is read at 400 dpi, and a pixel with a size of 42.3 μm is read at 600 dpi. When dust or a scar is present on the scan flow glass member 104, even if it is as very small as several tens μm, a streak is formed on a read image or a copy image formed from the read image because of the dust or scar described above.
Since the optical system is fixed, in the scan flow mode, the same one position on the scan flow glass member 104 is always read. When dust or a scar is present on this position, this problem also occurs.
With the ordinary original glass plate 101, even if dust or a scar is present on it, when a read image is formed or a copy image is formed from the read image, a single image degraded point of as small as several tens μm is merely formed. As described above, in the case of the scan flow mode, when dust or a scar is present at the reading position, it forms a streak from the leading edge to the trailing edge of the image, making the image very bad-looking. Accordingly, the image reading apparatus like that of the prior art is imposed a very heavy load in terms of the manufacture of the scan flow glass plate at the factory, management of dust or scar at the time of shipping, cleaning on the market, and the like.